Forecasting strategies

There are many ways of getting predictions for astronomical weather and there isn’t one single method that will always give accurate results. If things don’t pan out as predicted, the next best option is to adapt your strategy to what you’ve been offered.

For high-resolution Solar System work, good seeing is critical. At high altitude within the atmosphere, this is at the mercy of the jet stream. Services such as NetWeather (www.netweather.tv) provide constantly updated jet stream forecasts that will help you plan your observing. If the jet stream is raging over your location, chances are the seeing will be poor.

If you’re a solar observer, the Sun introduces its own special considerations. As the Sun rises, its light bathes the ground, causing it to warm up. The heat released causes thermal instability, which stabilises as the ground and air temperatures equalise. Towards the end of the day, energy absorbed by the ground is released as heat which destabilises the view, so picking the optimal time for solar observing requires keeping a record of the best time of day to begin.

Sometimes location just works against you. If you live to the east of a hill range, with the prevailing wind coming from the west, your location to the lee, or sheltered, side of the hills will likely promote instability and this will create poor mid-level seeing. You might want to find a new observing site on the other side of the hills, or keep watch for days when the wind is coming from a more suitable direction.

Looking at synoptic charts showing pressure, fronts and wind speeds will give you an idea of the weather and airflow over your observing site. Low pressure isn’t ideal as it tends to introduce clouds and disturbed, windy conditions. High pressure can produce clear and stable conditions, but these are not hard and fast rules and exceptions do happen. High pressure with fog can lead to frustration, although observing a planet or the Moon at high altitude can still give stable views.

Cloud forecasts are difficult to get right, especially at night. usee these as a guide rather than an accurate prediction of what you’re going to get. Monitoring infrared satellite images will give you a much better idea as to the viability of such forecasts and enable you to take advantage of unexpected cloud gaps.

Late morning, the ground and air temperatures reach a degree of equilibrium, which results in the best seeing of the day At its highest position due south, the Sun continues to impart energy to the ground and inequalities between air and ground temperature once again create instability; seeing tends to degrade Just after sunrise the ground begins to warm, causing the air layer above it to rise and exhibit turbulence. Vertically inclined surfaces facing the Sun warm quicker than horizontal ones ▼ The solar-seeing cycle timing tends to be unique for each location Late afternoon, the ground continues to release energy absorbed during the first half of the day; instability tends to persist leading to moderate or poor seeing Around sunset there is often a significant difference between ground and air temperature, which results in poor seeing

▲ Energy from the Sun greatly influences seeing conditions throughout the day